I have a laptop ibm thinkpad t21 the mother board is gone, operating system is win 2000 pro. I also have an ibm a20m with win98se. This laptop works fine but has a small hard drive. Can I swap the t21 hard drive into the a20m machine?

Maybe. Get the part number on the drive and go toibm.com and check the specs for the a20m to see what drives are compatible.

I have a laptop ibm thinkpad t21 the mother board is gone, operating system is win 2000 pro. I also have an ibm a20m with win98se. This laptop works fine but has a small hard drive. Can I swap the t21 hard drive into the a20m machine?
Buy external Hard Drive. Its cheap and I will help your from avoiding damaging your Motherboard again.